1. Field of the Invention
The present invention relates to gas turbine engines, and more particularly to a compressor impeller shroud construction for a gas turbine engine.
2. Description of the Prior Art
The compressor impeller in a Pratt & Whitney PT6 engine is the last compressor stage, being downstream of a series of alternating axially arranged compressor rotors and stators. It has an axial component and a radial component with the outlet portion of the impeller discharging the air so compressed radially into a tangential annular diffuser. The impeller hub is mounted to the compressor shaft while the impeller blades are unshrouded and are integral at their respective bases with the hub. The hub which supports the blades has a portion which is almost cylindrical and is massive at its inlet end and spreads out to a much thinner radial plate at its outlet end.
A static impeller shroud is conventionally mounted to the case of the engine in the compressor section by means of a support ring fixed to the case.
In the PT6 engine, struts extend across the gas path at the air intake supporting the outer case against the inner case which supports a thrust bearing. The thrust bearing is the support for the shaft and thus all the rotating parts of the compressor including the impeller. In other words, the only common datum for the tips of the impeller blades and the impeller shroud, which must be kept at close tolerances thereto, is the thrust bearing. Thus, thermal expansion grows from the thrust bearing for both the impeller blade tips and the impeller shroud. However, the path of expansion growth for the impeller shroud is much greater than for the impeller tips. The expansion growth for the impeller is directly through the rotor assembly to the thrust bearing, while the expansion growth for the impeller shroud is through the outer case.
Furthermore, it has been found that the outer periphery of the impeller can be distorted as a result of centrifugal forces due to its high rotational speed. Since the impeller shroud is fixed to the diffuser, it will remain static while the tips of the impeller blades will tend to reduce the clearance between the shroud and the blade tips. Furthermore, in a typical PT6 engine, the diffuser is fixed to a diaphragm structure which extends from the casing through the diffuser to a bearing on the gas generator side of the engine. During surge conditions, the pressure differential across this diaphragm structure causes axial deflection of the impeller shroud relative to the impeller blades in a direction opposite from the mechanical deflection of the impeller blades due to centrifugal forces.